To determine which of the given options is a sink for carbon monoxide (CO), we need to understand the role of each option in the context of CO absorption or conversion.$\\$• Option 1: Microorganisms present in the soil$\\$Microorganisms in the soil can act as a sink for CO. Certain soil bacteria have the ability to oxidize CO, converting it into carbon dioxide (CO$_2$), which is less harmful. This process helps in reducing the concentration of CO in the atmosphere.$\\$• Option 2: Oceans$\\$Oceans are not significant sinks for CO. While they absorb various gases, CO is not readily absorbed by ocean water in significant amounts.$\\$• Option 3: Plants$\\$Plants primarily absorb carbon dioxide (CO$_2$) during photosynthesis, not carbon monoxide (CO). Therefore, they are not considered a sink for CO.$\\$• Option 4: Haemoglobin$\\$Haemoglobin binds with CO to form carboxyhaemoglobin, which is harmful as it reduces the oxygen-carrying capacity of blood. Thus, haemoglobin is not a sink for CO; rather, it is adversely affected by CO.$\\$Based on the above analysis, the correct answer is Option 1: Microorganisms present in the soil, as they can convert CO into CO$_2$, thereby acting as a sink for CO.$\\$Therefore, the correct option is Option 1.